Plastic formulation and method for the production of plastic bottles in a two-stage stretch blow-molding process

ABSTRACT

An exemplary plastic formulation is disclosed for the production of plastic bottles in a two-stage stretch blow-molding process, and includes at least 60% of HDPE (High Density Polyethylene) having a density of 0.941 g/cm 3  to 0.965 g/cm3 and according to ISO 1133 a melt index 190° C./2.16 kg of 0.1 to 0.9 g/10 min. The HDPE contains a mono-modal or multi-modal HDPE having a second melt index 190° C./21.6 kg of 5 g/10 min to 50 g/10 min according to ISO 1133. A method is disclosed for producing a plastic bottle in a two-stage stretch blow-molding process using the plastic formulation.

RELATED APPLICATIONS

This application claims priority as a continuation application under 35U.S.C. §120 to PCT/EP2009/005126, which was filed as an InternationalApplication on Jul. 15, 2009 designating the U.S., and which claimspriority to Swiss Application 1161/08 filed in Switzerland on Jul. 24,2008. The entire contents of these applications are hereby incorporatedby reference in their entireties.

FIELD

The disclosure relates to method and a plastic formulation for producingplastic bottles in a two-station stretch blow-molding

BACKGROUND INFORMATION

A large number of plastic bottles and similar plastic containers usednowadays are produced in a stretch blow-molding process. In exemplarymethods, a so-called preform, which has an elongated tube-like shape onits one longitudinal end, has a bottom and on the other longitudinal enda neck region with molded threaded sections or the like, is insertedinto the mold cavity of a blow mold and is inflated by a medium whichhas been blown in with overpressure. Here the preform is additionallystretched in the axial direction with a stretching mandrel which hasbeen inserted through the neck opening. After thestretching/blow-molding process the finished plastic bottle is removedfrom the blow mold.

The single-layer or multi-layer preform is produced before the stretchblow-molding process in a separate injection molding method or in aplastic flow molding method. In the so-called single-station stretchblow-molding process the preform is inflated into a plastic containerimmediately after its production and stretched. But often the plasticcontainers are produced separately in space and time from the stretchblow-molding process in a two-station method and are temporarily storedfor later use. In the later stretch blow-molding process the preformsare heated again in order to produce plastic bottles from them. In thisway the two processes, injection molding and stretch blow-molding, canbe performed separately and optimally. In the stretch blow-moldingprocess the preform is brought to the specified temperature for exampleby infrared radiation and is stretched in the axial direction with astretching rod during the forming process and is radially molded in themolding tool by overpressure.

The raw material for producing plastic bottles in a stretch blow-moldingprocess is mainly polypropylene or PET (polyethylene terephthalate).Polypropylene and PET have been tested many times and their propertieshave been known for a long time. But due to its low stiffnesspolypropylene can have relatively poor top load properties. The creepproperties of oriented polypropylene are also poor.

Known plastic bottles which are produced in a two-station method, due tothe PET which is conventionally used and due to their wide range ofapplication, can engender major difficulties in the recycling stream.Straight plastic containers which are used for example as milk bottles,for preparing cosmetics or for storage of detergents are to be separatedin recycling since they are undesirable for direct contact with food.Due to the density of the PET, these bottles however cannot be separatedin a floating-sinking process.

For technical and economic reasons it would therefore be desirable ifother plastics which are matched to the specific problem, for exampleHDPE (high density polyethylene) could be processed in a stretchblow-molding process. HDPE also has relatively high stiffness even atlow wall thicknesses.

In the injection blow-molding process and in the single-station stretchblow-molding process, the use of HDPE as the raw material which is knownfrom the extrusion blow-molding process for producing plastic bottles isalready a popular practice. In the two-station stretch blow-moldingprocess with the high stretching ratios which are used there, HDPE hasnot been used to date since the poor stretch compaction of the HDPEeither did not enable any bottle production at all or resulted inplastic bottles with an overly dramatically fluctuating process or anonuniform wall thickness profile. Issues also arise fundamentally ininjection blow-molding and in a single-station stretch blow-moldingprocess with HDPE. But this can be managed by the preforms being allowedto expand only very little during inflation in order to solve issueswith unstable wall thickness. The preforms used in injectionblow-molding and in the single-station stretch blow-molding processtherefore already have a length which differs only a little from thelength of the bottles which have been produced. Accordingly thelongitudinal stretching ratio is only 1 to 1.8. The diameter stretchingratio is between 1 and at most 2.2.

The material distribution in known stretch blow-molding can beinfluenced via the viscosity of the raw material used. But those viscousraw materials which can be easily processed in the stretch blow-moldingprocess often have a viscosity which is too high for the injectionmolding process. Raw materials which are to be suitable for thetwo-station stretch blow-molding process should however satisfy thespecifications of the injection molding process for producing thepreforms and those of the stretch blow-molding process in which plasticbottles are produced from the preforms.

SUMMARY

A plastic formulation is disclosed for producing plastic bottles in atwo-station stretch blow-molding process, from a plastic preform,produced from an injection molding process or a flow molding process,for a plastic bottle producible in a separate stretch blow-moldingprocess, the preform being stretchable axially and radially, and theplastic formulation comprising: at least 60% HDPE (high densitypolyethylene) of a density from 0.941 g/cm³ to 0.965 g/cm³ and with afirst melt index 190° C./2.16 kg from 0.1 to 0.9 g/10 min according toISO 1133, the HDPE comprising a monomodal or multimodal HDPE with asecond melt index 190° C./21.6 kg from 5 g/10 min to 50 g/10 minaccording to ISO 1133.

A method is disclosed for producing a plastic bottle in a two-stationstretch blow-molding process, comprising: producing a preform from aplastic formulation in an injection molding process or in a flow moldingprocess from a plastic formulation; and producing a plastic bottleseparately from the preform in space and/or time, in a stretchblow-molding process, wherein the preform is axially and radiallystretched, and wherein the plastic formulation is based on an at leastbimodal HDPE containing at least 60% HDPE (high density polyethylene) ofa density from 0.941 g/cm³ to 0.965 g/cm³ and with a first melt index190° C./2.16 kg from 0.1 to 0.9 g/10 min according to ISO 1133, the HDPEcomprising a monomodal or multimodal HDPE with a second melt index 190°C./21.6 kg from 5 g/10 min to 50 g/10 min according to ISO 1133.

DETAILED DESCRIPTION

Exemplary embodiments are directed to preparation of a plasticformulation based on HDPE which is suitable for the two-station stretchblow-molding process. The HDPE formulation will allow a frictionlessinjection molding process without melt rupture, and on the other handenable sufficient stretch compaction in the stretch blow-moldingprocess. Furthermore a two-station stretch blow-molding process isdisclosed for processing of the plastic formulation.

An exemplary plastic formulation is disclosed for producing plasticbottles in a two-station stretch blow-molding process which is at least60% based on HDPE (high density polyethylene) which has a density from0.941 g/cm³ to 0.965 g/cm3 and a first melt index 190° C./2.16 kg from0.1 to 0.9 g/10 min according to ISO 1133, the HDPE comprising amonomodal or multimodal HDPE with a second melt index 190° C./21.6 kgfrom 5 g/10 min to 50 g/10 min, likewise measured according to ISO 1133.Preferably the plastic formulation has a first melt index of, forexample, 190° C./2.16 kg of roughly 0.3 g/10 min and a second melt indexof, for example, 190° C./21.6 kg of 30 g/10 min and a bimodaldistribution.

The plastic matrix which is based on bimodal or multimodal HDPE isoptimized on the one hand for the injection molding process (or for aflow molding process) and on the other hand is also well suited to thestretch blow-molding process. The HDPE of chosen density can beprocessed largely without problems in the injection molding process. Forexample, in the injection molding process the viscosity of the HDPE witha low molecular weight provides for melt ruptures being prevented. TheHDPE with a greater molecular weight is, for example, responsible forsufficient stretch compaction of the plastic being achieved in thestretch blow-molding process. The shrinkage values which can be achievedwith the plastic formulation based on multimodal HDPE correspond largelyto that of PET shortly after injection molding. This makes it possibleto use injection molding molds from PET processing in the injectionmolding process. The analogous applies to the stretch blow-moldingprocess, where the existing blow molds for PET processing can likewisecontinue to be used. Thus, switching of the raw material from PET to aplastic formulation based on HDPE does not necessarily result in toolinvestments.

An exemplary HDPE formulation as disclosed makes it possible to alsoproduce plastic bottles in a two-station stretch blow-molding processand in doing so to use the advantages of HDPE, for example its highwater vapor barrier relative to the PET of the plastic bottles producedfrom it. Plastic bottles of the HDPE formulation as disclosed herein atlower bottle weight have comparable strength values to PET bottles.Lower weight means lower raw material use and thus lower petroleumconsumption and conservation of resources as well as a reduction of CO₂release. This makes the use of an exemplary HDPE plastic formulation asdisclosed advantageous both from the economic and also the ecologicalstandpoint. A plastic formulation which is produced essentially (i.e.,substantially) based on bimodal or multimodal HDPE has a density from0.941 g/cm³ to 0.965 g/cm³. In this way the plastic bottles which havebeen produced from the plastic formulation can be very easily sorted outfrom the PET stream, for example in a floating-sinking method, by way oftheir density.

For economic reasons, up to 40% calcium carbonate can be added to theplastic formulation in order to reduce the specified amount of plastic.

One version of the disclosure has at least 90% HDPE by weight. Foreconomic reasons this version of the plastic formulation also cancontain up to 10% by weight calcium carbonate and/or talc and/or amixture of polymers which are suited for the blow molding process asfiller. The addition of these types of fillers allows changes in thedensity and the rheological properties only to occur to a very limiteddegree so that the processing parameters need not be adapted at all oronly to a minor degree. The attainable strength values of the plasticbottles which have been produced in the two-station stretch blow-moldingmethod are only insignificantly influenced by adding fillers in theindicated amounts.

For producing opaque plastic bottles, 0.1% to 4% of a dye based ontitanium oxide and/or zinc oxide can be added to the plasticformulation.

An exemplary HDPE which is used in the plastic formulation as disclosedherein comprises a bimodal or multimodal HDPE with a first melt indexof, for example, 190° C./2.16 kg of 0.1 g/10 min to 0.9 g/10 min.Furthermore a second melt index 190° C./21.6 kg from 5 g/10 min to 50g/10 min measured according to ISO 1133 is considered characteristic.Preferably the plastic formulation has an exemplary first melt index of190° C./2.16 kg of roughly 0.3 g/10 min and a second exemplary meltindex of 190° C./21.6 kg of 30 g/10 min and a bimodal distribution.

An exemplary plastic matrix which is based on HDPE can be optimized onthe one hand for the injection molding process (or for a flow moldingprocess) and on the other hand can be also well suited to the stretchblow-molding process. Here, in the injection molding process, theviscosity of the exemplary HDPE with a low molecular weight provides formelt ruptures being prevented. The HDPE with a greater molecular weightis essentially responsible for sufficient stretch compaction of theplastic being achieved in the stretch blow-molding process. Theshrinkage values which can be achieved with the plastic formulationbased on multimodal HDPE correspond largely to those of PET shortlyafter injection molding. This makes it possible to use injection moldingmolds from PET processing in the injection molding process. Theanalogous applies to the stretch blow-molding process, where theexisting blow molds for PET processing can likewise continue to be used.Thus, switching of the raw material from PET to a plastic formulationbased on HDPE does not necessarily result in tool investments.

In an exemplary method as disclosed herein for producing a plasticbottle in a two-station stretch blow-molding process, first of all in aninjection molding process or in a flow molding process a preform isproduced from the plastic formulation and from it a plastic bottle isproduced in a stretch blow-molding process in a second process stepwhich is separate in time and/or space. In doing so the preform isaxially and radially stretched. In the method as disclosed herein, theplastic formulation which is based on, for example, at least 60% HDPE ofa density from 0.941 g/cm³ to 0.965 g/cm³ and an exemplary melt index190° C./2.16 kg of 0.1 to 0.9 g/10 min according to ISO 1133 isprocessed.

Other exemplary versions of the method call for the plastic formulationswhich have been modified according to the above described developmentsto be used.

The use of the HDPE plastic formulations as disclosed herein makes itpossible to carry out a “normal” two-station stretch blow-moldingprocess in which the preform produced from the raw material is axiallystretched and radially stretched to a sufficient degree in order toimpart the specified strength values to the plastic bottle which hasbeen produced.

In order to avoid unduly high injection pressures and long residencetimes of the plastic formulation in the injection molding tool in theproduction of the preform, the preform can be produced with an injectionpoint which has a diameter from, for example, 2 mm to 5 mm. Exemplaryinjection pressures in the hot channel are between 200 bar and 2000 bar.

In one process version the preform is produced with an exemplary averagewall thickness from 2.1 mm to 2.9 mm. At these wall thicknesses thepreform can be brought to the specified processing temperature ratherquickly and uniformly in the stretch blow-molding process and thedesired high processing speeds can be achieved without adverse effectson quality with respect to dimensional stability or strengths.

One version of the disclosure calls for the preform to be heated to anexemplary temperature of 115° C. to 135° C. for processing in thestretch blow-molding process. For example, the low temperatures areinherently atypical for HDPE. The special HDPE plastic formulationhowever allows these temperature ranges in which the flow behavior ofthe HDPE is especially regular, and accordingly even more complexlyshaped plastic containers can be inflated in the desired quality. Hereit is feasible if the preform is heated to a processing temperature of,for example, roughly (e.g., ±10%) 125° C. for further processing in theblow mold. This takes place for example by infrared radiation.

So that the manufactured plastic bottle reaches top load values as highas possible, one process version calls for the preform in the stretchblow-molding process to be axially stretched in a longitudinalstretching ratio from, for example, 1.8 to 3.2 and thus acquire highstrength.

Without stretch compaction, less stretched regions of the bottles wouldhave very thick walls and those regions of the bottle which are verydramatically stretched would have very thin walls. Stretch compaction ofthe selected HDPEs makes it possible to largely reduce this effect andto draw material from the thick sites of the bottle into the corners andedges. Oval HDPE bottles with a width to depth ratio from, for example,1 to 3 thus become possible; this enables production of distinctly moreoval bottles, for example in plastic bottles for cosmetic applicationsor for detergents.

The axial stretching rate is chosen to be, for example, 0.8 m/s to 2 m/sin order on the one hand to achieve cycle times as short as possible forthe production process and on the other hand to ensure that thestretching rod does not axially penetrate the preform which has beenheated to the processing temperature.

So that the manufactured plastic bottle also has sufficient strength inthe circumferential direction, in one version of the disclosure aneffort is made to radially stretch the preform in the stretchblow-molding process in a diameter stretching ratio from, for example,2.4 to 3.6.

The actual blow-molding process in the stretch blow-molding process canadvantageously take place with exemplary preblowing pressures of 1 bar-5bar and with exemplary main blowing pressures from 5 bar to 15 bar. Thiscan allow fast production cycles with simultaneously careful treatmentof the raw material.

The preform in the stretch blow-molding process is inflated and axiallystretched such that an inflated round bottle has a fluctuation of itswall thickness which is less than, for example, ±0.5 mm. With thesesmall fluctuation widths the wall thickness of the plastic bottles canbe further optimized without falling below the allowable minimum wallthicknesses which are involved for the specified axial and radialstrengths of the HDPE bottles. Reduced raw material use means lesspetroleum consumption and conservation of resources as well as areduction of CO₂ release. All this makes the use of the HDPE plasticformulation as disclosed herein and the two-station stretch blow-moldingprocess which has been optimized for this purpose desirable both from aneconomic and also ecological standpoint.

Thus, it will be appreciated by those skilled in the art that thepresent invention can be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Thepresently disclosed embodiments are therefore considered in all respectsto be illustrative and not restricted. The scope of the invention isindicated by the appended claims rather than the foregoing descriptionand all changes that come within the meaning and range and equivalencethereof are intended to be embraced therein.

1. Plastic formulation for producing plastic bottles in a two-stationstretch blow-molding process, from a plastic preform, produced from aninjection molding process or a flow molding process, for a plasticbottle producible in a separate stretch blow-molding process, thepreform being stretchable axially and radially, and the plasticformulation comprising: at least 60% HDPE (high density polyethylene) ofa density from 0.941 g/cm³ to 0.965 g/cm³ and with a first melt index190° C./2.16 kg from 0.1 to 0.9 g/10 min according to ISO 1133, the HDPEcomprising a monomodal or multimodal HDPE with a second melt index 190°C./21.6 kg from 5 g/10 min to 50 g/10 min according to ISO
 1133. 2.Plastic formulation as claimed in claim 1, comprising: 0% to 40% calciumcarbonate as a filler.
 3. Plastic formulation as claimed in claim 1,comprising: a proportion of HDPE in the plastic formulation of at least90%.
 4. Plastic formulation as claimed in claim 3, comprising: 0%-10%calcium carbonate and/or talc and/or a mixture of polymers which aresuitable for a blow molding process as a filler.
 5. Plastic formulationas claimed in claim 1, comprising: 0% to 4% of a dye based on titaniumoxide and/or zinc sulfide.
 6. Plastic formulation as claimed in claim 1,comprising: HDPE with a first melt index of 190° C./2.16 kg of roughly0.3 g/10 min and a second melt index of 190° C./21.6 kg of 30 g/10 minand a bimodal distribution.
 7. Method for producing a plastic bottle ina two-station stretch blow-molding process, comprising: producing apreform from a plastic formulation in an injection molding process or ina flow molding process from a plastic formulation; and producing aplastic bottle separately from the preform in space and/or time, in astretch blow-molding process, wherein the preform is axially andradially stretched, and wherein the plastic formulation is based on anat least bimodal HDPE containing at least 60% HDPE (high densitypolyethylene) of a density from 0.941 g/cm³ to 0.965 g/cm³ and with afirst melt index 190° C./2.16 kg from 0.1 to 0.9 g/10 min according toISO 1133, the HDPE comprising a monomodal or multimodal HDPE with asecond melt index 190° C./21.6 kg from 5 g/10 min to 50 g/10 minaccording to ISO
 1133. 8. Method as claimed in claim 7, comprising:producing the preform with an injection point which has a diameter from2 mm to 5 mm; and injecting the preform with pressures between 200 barand 2000 bar in a hot channel.
 9. Method as claimed in claim 7,comprising: producing the preform with an average wall thickness from2.1 mm to 2.9 mm.
 10. Method as claimed in claim 7, comprising: heatingthe preform to a temperature from 115° C. to 135° C. for processing inthe stretch blow-molding process.
 11. Method as claimed in claim 7,comprising: axially stretching the preform in the stretch blow-moldingprocess in a longitudinal stretching ratio from 1.8 to 3.2.
 12. Methodas claimed in claims 11, wherein an axial stretching rate is 0.8 m/s to2 m/s.
 13. Method as claimed in claim 7, comprising: radially stretchingthe preform in the stretch blow-molding process in a diameter stretchingratio from 2.4 to 3.6.
 14. Method as claimed in claim 7, comprising:inflating, and axially stretching, the preform in the stretchblow-molding process such that an inflated bottle, in areas betweencorners and edges, has a fluctuation of its wall thickness which issmaller than +0.5 mm.
 15. Plastic formulation as claimed in claim 4,comprising: 0% to 4% of a dye based on titanium oxide and/or zincsulfide.
 16. Plastic formulation as claimed in claim 15, comprising:HDPE with a first melt index of 190° C./2.16 kg of roughly 0.3 g/10 minand a second melt index of 190° C./21.6 kg of 30 g/10 min and a bimodaldistribution.
 17. Method as claimed in claim 8, comprising: producingthe preform with an average wall thickness from 2.1 mm to 2.9 mm. 18.Method as claimed in claim 17, comprising: heating the preform is heatedto a temperature from 115° C. to 135° C. for processing in the stretchblow-molding process.
 19. Method as claimed in claim 18, comprising:axially stretching the preform in the stretch blow-molding process in alongitudinal stretching ratio from 1.8 to 3.2.
 20. Method as claimed inclaim 19, comprising: radially stretching the preform in the stretchblow-molding process in a diameter stretching ratio from 2.4 to 3.6.